The present application relates to power converters, and more particularly, to methods for controlling power-packet-switching power converters, especially in standalone environments such as microgrid applications.
Note that the points discussed below may reflect the hindsight gained from the disclosed inventions, and are not necessarily admitted to be prior art.
A new kind of power converter was disclosed in U.S. Pat. No. 7,599,196 entitled “Universal power conversion methods,” which is incorporated by reference into the present application in its entirety. This patent describes a bidirectional (or multidirectional) power converter which pumps power into and out of a link inductor which is shunted by a capacitor.
The switch arrays at the ports are operated to achieve zero-voltage switching by totally isolating the link inductor+capacitor combination at times when its voltage is desired to be changed. (When the inductor+capacitor combination is isolated at such times, the inductor's current will change the voltage of the capacitor, as in a resonant circuit. This can even change the sign of the voltage, without loss of energy.) This architecture has subsequently been referred to as a “current-modulating” or “Power Packet Switching” architecture. Bidirectional power switches are used to provide a full bipolar (reversible) connection from each of multiple lines, at each port, to the rails connected to the link inductor and its capacitor. The basic operation of this architecture is shown, in the context of the three-phase to three-phase example of patent FIG. 1, in the sequence of drawings from patent FIG. 12a to patent FIG. 12j. 
The ports of this converter can be AC or DC, and will normally be bidirectional (at least for AC ports). Individual lines of each port are each connected to a “phase leg,” i.e. a pair of switches which permit that line to be connected to either of two “rails” (i.e. the two conductors which are connected to the two ends of the link inductor). It is important to note that these switches are bidirectional, so that there are four current flows possible in each phase leg: the line can source current to either rail, or can sink current from either rail.
Many different improvements and variations are shown in the basic patent. For example, variable-frequency drive is shown (for controlling a three-phase motor from a three-phase power line), DC and single-phase ports are shown (patent FIG. 21), as well as three- and four-port systems, applications to photovoltaic systems (patent FIG. 23), applications to Hybrid Electric vehicles (patent FIG. 24), applications to power conditioning (patent FIG. 29), half-bridge configurations (patent FIGS. 25 and 26), systems where a transformer is included (to segment the rails, and allow different operating voltages at different ports) (patent FIG. 22), and power combining (patent FIG. 28).
Improvements and modifications of this basic architecture have also been disclosed in U.S. Pat. Nos. 8,391,033, 8,295,069, 8,531,858, and 8,461,718, all of which are hereby incorporated by reference.
The term “converter” has sometimes been used to refer specifically to DC-to-DC converters, as distinct from DC-AC “inverters” and/or AC-AC frequency-changing “cycloconverters.” However, in the present application the word converter is used more generally, to refer to all of these types and more, and especially to converters using a current-modulating or power-packet-switching architecture.
Microgrids are basically self-contained electrical ecosystems. Microgrid generation resources can include fuel cells, wind, solar, or other energy sources. Power is produced, transmitted, consumed, monitored, and managed all on a local scale. In many cases, they can be integrated into larger, central grids, but their defining characteristic is that they can operate independently if disconnected from the whole.
When operating independently from larger central grids, microgrid applications must generate and control voltage to supply the loads contained inside the microgrid. A microgrid can include a power converter, which generally converts direct current obtained from the generation sources into alternating current that can be fed to the load. In a microgrid working independently from larger central grids, a power converter can work in standalone mode, meaning that the converter can work as a power generator creating and controlling output current entirely on its own, unlike a grid-tie converter where the current is directly fed into a utility grid with an active output voltage. For transferring energy from an input to an output in a power converter, common control modes can generally include constant voltage control mode, constant current control mode and constant power control mode, among others.
These control modes can present certain shortcomings, such as instability and lack of flexibility when a change either in current or in voltage occurs. For example, in a constant power control mode, when voltage drops, current tends to go up to maintain a constant power. This rise in current can drop voltage even further, which can create a current overshoot that can result in system shut-off. Likewise, if there is an increase in voltage, current can tend to decrease to maintain constant power, which can create oscillations that can also result in system shut-off and/or damages in the power converter.